Retroreflectors are used as reflective devices for marking articles and persons, the surface or clothing respectively of which greatly absorbs light and is therefore difficult to see in the dark, for example at night, even when strong lighting is involved. The retroreflectors are provided with reflective surfaces which are enclosed in a layer composite and are applied by means of adhesive for example to predetermined surfaces. In that way retroreflectors enhance the visibility of for example building barriers, optical markings, bicycles, motor vehicles and so forth.
Reference is made in this respect to U.S. Pat. No. 3,712,706 disclosing a retroreflector in which a relief structure is composed of reflective tetrahedron elements in the form of corner cubes. Production of the relief structure with the periodically arranged tetrahedron elements is effected by cutting three sets of parallel, V-shaped grooves into the surface of a metal master plate, by means of a diamond tool. The three sets with the parallel, V-shaped grooves intersect at predetermined angles in such a way that the surface of the metal master plate has the relief structure formed from the periodically arranged tetrahedron elements. Retroreflectors of that kind reflect incident light in a narrow spatial angle centered around the direction of the incident light. An increase in the angle of the reflected light is effected by diffraction of the light at the relief structure.
WO 00/43813 describes a retroreflector which in substance corresponds to U.S. Pat. No. 3,712,706 as just discussed above. Chips or small portions of such retroreflectors can be added as a pigment to a lacquer. An article which is coated with such a lacquer has a strongly reflective surface.
U.S. Pat. No. 1,671,086 provides the teaching of not excessively polishing the reflective surfaces of the retroreflector with the typical cube edge, so that the reflected light can be scattered at the minor irregularities in the reflective surfaces of the retroreflector. The parallel light beam which is incident on the retroreflector is reflected and is returned in a cone with the incident light beam as the axis in a symmetrical condition in expanding relationship back to the light source.
Attention may be directed here to DE 44 29 683 C1 describing a triple reflector which is a body formed from structure elements, each of which is of a shape delimited by three reflective square faces. The three faces form a surface which corresponds to the cube surface which is visible in the direction of the cube diagonals. The three faces of each structure element can be slightly curved inwardly in the manner of a hollow mirror and may have a grooving so that the triple reflector reflects the incident parallel light beam back to the light source in divergent relationship.
Reference may also be made to U.S. Pat. No. 4,588,258 and U.S. Pat. No. 4,938,563 in which shaping the relief structure of a retroreflector in a flexible film material, by virtue of the small thickness of material involved, necessarily results in microscopically small lateral dimensions of the elements of the relief structure and consequently affords strong light diffraction effects. Minor asymmetries in the relief structure, which incline the axes of the tetrahedron elements through a few degrees of angle out of the normal direction, reduce the scatter of the reflected light. U.S. Pat. No. 4,938,563 more specifically refers to the possibility of influencing divergence or scatter of the reflected light by suitably arranging the tetrahedron elements, of differing asymmetrical nature, within the relief structure.
DE 696 19 691 T2 provides that the retroreflectors can be improved by a diffraction structure which is superimposed on the relief structure and which imparts to the reflected light a color which is dependent on the spatial frequency of the diffraction structure, without the use of colored pigments.
A summary of materials which are suitable for the production of layer composite material for retroreflectors is to be found in U.S. Pat. No. 4,856,857.
It will be appreciated that retroreflectors are readily commercially available. An aspect in common to such retroreflectors is that it is not possible to derive any conclusions about the orientation of the retroreflector from the reflected light.